The present invention is generally directed to devices used in industrial septic processing and, more specifically, to a mechanically seal-less magnetically driven scraped-surface heat exchanger.
Scraped-surface heat exchangers are commonly utilized in aseptic processing of foodstuffs. These heat exchangers are preferred because of their capability to process heat-sensitive, viscous, or particulate-laden products, enhance the heat transfer of viscous products, and minimize the extent of burn-on, or fouling on the heat transfer surface. Such heat exchangers are commonly marketed under the trade names, for example, Votator(copyright), Thermutator(copyright), Contherm(copyright) and Terlotherm(copyright). Waukesha Cherry-Burrell, Delavan, Wis., for example, manufactures such heat exchangers.
FIG. 1 illustrates the basic operating principles of a scraped surface heat exchanger. In particular, a scraped surface heat exchanger 12 generally consists of mutator shaft 13 that rotates within a heat transfer tube 14. Foodstuff passes though through an annulus 15 formed between the shaft and the heat transfer tube. A heating or cooling medium generally flows through a jacket 16 formed about the heat transfer tube, while insulation 17 surrounds the jacket to minimize energy heat loss. Generally a stainless steel cover 18 protects the insulation and forms the outer housing. In operation, the rotating shaft carries a series of staggered blades 19 that continuously scrape product film from the heat transfer tube wall. The xe2x80x9ccleanedxe2x80x9d wall thus, enhances heat transfer, and produces a homogenous of foodstuff passing through the heat exchanger.
It is desirable for the entire rotating shaft assembly to be able to be easily removed for inspection and maintenance. Typically a scraped-surface heat exchanger is designed with two boltless V-lock heads, one at each end of the heat exchangers. The boltless V-lock at the opposite drive head end contains a frictionless ball-bearing to support the rotating shaft and a rotary mechanical seal in direct contact with the product inside the heat exchanger. In contrast, the boltless V-lock at the drive head end encompasses the second rotary mechanical seal only. The corresponding second frictionless ball-bearing to support the rotating shaft is located inside the gear box of the mechanical drive.
A typical rotary (or dynamic) mechanical seal for a scraped-surface heat exchanger includes a seal head insert and a seal body insert, both contributing to the mechanical seal face. Standard seal faces consist of hardened surfaces like silicon carbide or chromium oxide against a special graphite compound. In aseptic processing, these mechanical seal faces serve both to maintain a mechanical seal (i.e., a pressure differential between the inside and outside of the heat exchanger) and an aseptic seal (i.e., an aseptic-safety barrier between the inside and outside of the heat exchanger). To ensure seal integrity, a mechanical seal face needs to be properly lubricated, kept free of foreign material, and maintained at a low temperature. For these reasons, a barrier fluid has to continuously flood the rotary mechanical seal. In aseptic processing, this barrier fluid must meet high purity and safety standards.
Nonetheless, the possibility that one of the mechanical parts of the two rotary mechanical seals of a traditional scraped-surface heat exchanger fails during operation is very high. Notably, under the operating conditions associated with aseptic processing, a mechanical failure of the rotary mechanical seal (which generally causes product leakage) can result in an aseptic failure. Thus, a need exists for a seal-less scraped-surface heat exchanger that is compatible with the requirement-s of aseptic processing of food products, such as puddings and gels.
The present invention is directed to a seal-less magnetically driven scraped-surface heat exchanger that is particularly useful for aseptic processing. By eliminating the mechanical seals used in traditional heat exchangers (with their numerous associated parts), the present invention reduces the possibility of mechanical failure at the ends of the heat exchanger.
In one embodiment, the invention is directed to a scraped-surface heat exchanger comprising an elongated generally cylindrical heat transfer tube having an inlet, an outlet, and a sidewall defining a chamber between the inlet and the outlet. An elongated media tube is provided in surrounding relation to the heat transfer tube. A rotary shaft is mounted axially within the heat transfer tube. The rotary shaft has an outer surface and one or more scraper blades extending from the outer surface of the rotary shaft. A drive end containment shroud is mounted at an axial end of the heat transfer tube. The drive end containment shroud has a closed end, an open end, and a sidewall defining a drive chamber in open communication with the interior chamber of the heat transfer tube through the open end of the containment shroud. An inner rotatable magnet assembly is mounted within the drive chamber of the drive end containment shroud and connected to the rotary drive shaft. An outer rotatable magnet assembly is mounted outside the drive end containment shroud and magnetically coupled to the inner rotatable magnet assembly. In use, rotation of the outer magnet assembly results in rotation of the inner magnet assembly, which results in rotation of the rotary drive shaft.
In a particularly preferred embodiment, the heat exchanger further comprises a second containment shroud mounted at an axial end of the heat transfer tube opposite the drive head end containment shroud. The second containment shroud has a closed end, an open end, and a sidewall defining a cavity in open communication with the interior chamber of the heat transfer tube through the open end of the second containment shroud. An axial magnetic bearing system is provided comprising an axial magnetic rotor coupled to the rotary shaft and contained within the second containment shroud, and an axial magnetic bearing stator mounted outside the second containment shroud. In use, the axial magnetic bearing stator generates an electromagnetic field to longitudinally align the axial magnetic rotor and rotary shaft in a desired position relative to the heat transfer tube. A radial magnetic bearing system is also provided comprising a radial magnetic rotor coupled to the rotary shaft and contained within the second containment shroud, and a radial magnetic bearing stator mounted outside the second containment shroud. In use, the radial magnetic bearing stator generates an electromagnetic field to radially align the radial magnetic rotor and rotary shaft in a desired position relative to the heat transfer tube.